During the 1980s retroviruses emerged as important agents in the etiology of cancers and other human diseases. Our studies of human T-lymphotrophic virus type I (HTLV-I) have documented that risk for adult T-cell leukemia is markedly elevated for HTLV-L positives and preliminary data from a case-control study show an absence of other factors previously associated with risk for non-Hodgkin's lymphoma, while supporting our previous conclusion that early life exposure is critical for disease occurrence. We reported for the first time that lymphocytes from HTLV-I seropositive, clinically healthy individuals undergo spontaneous lymphocyte proliferation when placed in tissue culture. A major new finding is the recognition that some American Indian populations may be a natural reservoir for HTLV-II infection, a virus previously associated with parenteral drug abuse. Our studies of HIV have documented important insights concerning the fact that older age individuals progress more rapidly to clinical AIDS. Intermediate markers such as CD4 count also drop more rapidly in older HIV-infected persons and such drops have a more ominous prognostic significance in older compared to younger persons. New data of relevance to vaccine development have emerged from studies of antibodies associated with maternal-to-child transmission. DNA tumor viruses are being evaluated as cofactors in HIV-1 related malignancy as well as the role of the newly discovered human herpes virus 6 as an oncogenic agent.